1. Field of the Invention
The present invention relates to a surface inspection system, such as a wafer inspection system. Specifically, the present invention relates to a method of using exposure control to improve defect sensitivity across the surface, which may contain multiple scattering regions.
2. Background of the Invention
Traditionally, there have been two scattering-based, patterned-wafer inspection systems: a laser spot scanning system and a flood-illumination imaging system. In general, the sensitivity of a system depends on three fundamental system parameters in patterned-wafer inspection, which comprise (1) spatial resolution, (2) Fourier filtering and (3) multi-channel filtering. Fourier filtering eliminates the repetitive or periodic scattering light intensity patterns from the wafer, and multi-channel filtering discriminates the shape of patterns from the wafer. The advantages of the laser spot scanning system are multi-channel filtering capability and Fourier filtering on the array, when the cell size is smaller than the spot size. The advantages of the flood-illumination imaging system are high spatial resolution and effective Fourier filtering capability.
Recently, aspects of a laser spot scanning system and a flood-illumination imaging system have been successfully combined in a xe2x80x9cline-illumination imaging systemxe2x80x9d (i.e., a hybrid system), as described in co-assigned PCT patent application Ser. No. PCT/US98/16116, filed on Jul. 28, 1998 and published on Feb. 11, 1999, which is hereby incorporated by reference in its entirety. The line-illumination imaging system provides spatial resolution that satisfies today""s desired market requirements and future market requirements, and retains Fourier filtering and multi-channel filtering capabilities.
In addition to the sensitivity of a wafer inspection system, high throughput may also be desired. The throughput of a system depends on the data rate of the system. A spot scanning system may use a Photo Multiplier Tube (PMT) for detection. A flood-illumination may use Time Delay Integration (TDI) for detection. A line-illumination imaging system may use one or more line-scan Charge Couple Devices (CCDs) for detection. A PMT processes one data point at a time, but a TDI or a CCD may process several data points simultaneously. Therefore, the data rates (and thus throughput) for a flood-illumination with TDI and a line-illumination system with CCD are usually higher than that of a spot scanning system with PMT.
Other than the system parameters described above, not utilizing the full dynamic range of a detector on some regions of the wafer during inspection can penalize detection sensitivity. The xe2x80x9cdynamic rangexe2x80x9d of a detector is the range between minimum and maximum detectable light of the detector. The scattering intensity of a pattern depends on many factors, such as for example, local wafer structures, illumination angle, illumination polarization, illumination wavelength, illumination line width, collection angle, collection polarization, and numerical aperture (NA) of a collector. Collection NA is the sine of the half angle of the cone of collection in this application. Within a die, there are random (logic) structures and array patterns. The logic structures scatter randomly, and the array structures scatter periodically. The periodic array patterns can be blocked out using Fourier filters, and the remaining scatter light intensity signal should be caused by defects. Therefore, the scattering light intensity may differ significantly from region to region within a die.
In order to obtain good sensitivity across the wafer, the scattering intensity collected by the detector should to be within a certain dynamic range or limit. If the scattering intensity is too strong, it will saturate the detector and give false counts after die-to-die or cell-to-cell comparison. If the scattering intensity is too low, a scattering light intensity signal caused by a defect on the wafer may be too weak to overcome the electronic noise after die-to-die or cell-to-cell comparison, and results in lost sensitivity. Therefore, the upper limit of the detector is determined by detector saturation, and the lower limit is determined by electronic noise.
Traditionally, the power of the laser and the integration time (time period to collect light) of the detector are fixed during inspection. Therefore, it is difficult to guarantee that all scattering intensities coming out of the wafer are within a certain dynamic range, and the system may lose sensitivity on some regions. To solve this issue, either the laser power or the detector""s integration time should change during inspection. It may not be practical to change the laser power due to the need for high-speed switching and other problems.
A method and system using exposure control to inspect a surface, such as a wafer, is provided in accordance with the present invention. This method utilizes the full dynamic range of a detector and provides good defect sensitivity across the surface, which contains uneven light scattering regions. In one embodiment, this method is implemented by a line-illumination imaging system with CCDs for detection.
The invention described above may be used to provide a viable alternate mechanism to inspect patterned or unpatterned wafers, photomasks, reticles, liquid crystal displays and other flat panel displays. Also, this invention may be used for any inspection system that uses CCDs for detection.
One aspect of the invention relates to a system configured to inspect a surface, such as a wafer. The surface comprises at least two regions that scatter light differently. The system comprises a charge coupled device (CCD) configured to collect light scattered from the surface. The CCD comprises a plurality of taps. Each tap comprises a set of pixels. The CCD is configured to independently adjust an integration time of each tap depending on where the set of pixels of the tap is positioned to collect light scattered from the surface.
Another aspect of the invention relates to a charge coupled device (CCD) configured to collect light scattered from a surface, such as a wafer. The surface comprises two regions that scatter light differently. The CCD comprises a plurality of taps. Each tap comprising a set of pixels. The CCD is configured to independently adjust an integration time of each tap depending on where the set of pixels of the tap is positioned to collect light scattered from the surface.
Another aspect of the invention relates to a method of collecting light scattered from a surface, such as a wafer. The surface comprises two regions that scatter light differently. The method comprises setting integration times for a plurality of taps in a charge coupled device (CCD) according to where pixels of the taps are positioned to collect light scattered from the surface; and collecting light scattered from the surface during the integration times.
Another aspect of the invention relates to a method of setting integration times for a plurality of taps in a charge coupled device (CCD). The CCD is configured to scan a surface, such as a wafer, comprising two regions that scatter light differently. The method comprises setting a first integration time for a first tap positioned to collect light scattered from the first region; setting a second integration time for a second tap positioned to collect light scattered from the second region, wherein the first integration time is different than the second integration time; and collecting light scattered from the first and second regions.
In one embodiment, collection optics are arranged such that radiation scattered from different parts of a line illuminated by a beam from a laser is imaged onto different pixels (and thus taps) of the same CCD.